Many investigations on thrombus formation have been directed toward the mechanisms involved in reactions of platelets to platelets, using in vitro techniques. This current investigation will be directed toward mechanisms involved in reactions of platelet aggregates and the vessel wall in the living animal. Direct microscopic observations of platelet aggregates in vivo will be made to determine what factors are responsible for platelet aggregate adherence to the wall. It has been established that overt vessel wall injury is not a prerequisite for platelet aggregation or platelet adhesiveness, but it is suspected that some interaction occurs between the aggregates and the wall that induces adherence. Parameters to be investigated include changes in normal characteristics of the endothelial lining of vessels, velocity of blood flow, and wall permeability following the production of platelet aggregates in bat wing vessels and hamster cheek pouch vessels.